1. Field of the Invention
The invention relates to substrate handling robots.
2. Description of Related Art
In the semiconductor and other industries, robots are called upon to perform a variety of tasks requiring high repeatability and precision. For example, in semiconductor wafer processing, cassettes containing a plurality of semiconductor wafers are loaded and unloaded into a micro-environment in which the wafers are to undergo processing. The loading and unloading functions involve automated motions performed by a robot, also serving to variously transport the wafers between different processing stations in the micro-environment. Such a robot is disclosed in co-pending U.S. patent application Ser. No. 09/079,850, entitled xe2x80x9cRobot Having Multiple Degrees of Freedomxe2x80x9d, incorporated herein by reference in its entirety.
The robot in the aforementioned patent application is of the type known as a Global Positioning Robot (GPR) and, as seen in FIG. 1, comprises a base unit 12 having one or more telescoping platforms 14 atop of which is mounted a robot arm 16 with an end effector 18 for handling the substrates. The telescoping motion constitutes motion in the Z axis, with the robot further adapted to tilt about the Z axis. Tilting is effected by independently actuating Z motion along means such as motors (not shown). Other tilting mechanisms are also known and may be used to effect the tilting along the Z axis.
The robot arm 16 is additionally capable of motion in a plane defined by R and xcex8 coordinates in a conventional cylindrical coordinate system such that the end effector 18 can move anywhere about a predetermined region in the plane, taking a variety of possible paths including both linear and non-linear paths. This motion is effected using appropriate actuation means such as motors and associated belt-pulley linkages (not shown) as described in the aforementioned patent application. Other motions include yaw and roll motion of the end effector 18, permitting the robot to achieve six or more degrees of freedom and possibly kinematic redundancy. The actuation means are controlled using a suitable control means such as a microprocessor adapted to issue the appropriate commands to achieve the desired motion trajectories.
The amount of precision which can be realized in robotic applications is dependent upon various factors and is limited by for example the geometry and stiffness of the moving components such as the robot arm. The weight of the substrate being manipulated by the robot also imparts certain deflections on the system, and with the advance of the semiconductor and LCD technology fields requiring the handling of larger and larger substrates, this factor becomes increasingly significant. As the substrate is transported between different positions by the robot, deviations from the intended path inevitably occur, compromising the accuracy of the system and imposing undesirable constraints, such as for example the need to increase spacing between the wafers in a cassette in order to accommodate expected deviations. Problems can thus arise due to inaccuracies or deflection of the robot arm, deflection of the end effector of the robot arm or of the manipulated substrate, or to misalignment of the substrate and/or cassette.
To better explain the problems encountered, an ideal situation will first be discussed. FIG. 2A shows the ideal case in which the substrate, in this case a semiconductor wafer 24 having a substantially planar shape and an object axis P lying in its primary plane, is centered within its designated slot 28 in cassette 22. The orientation of the wafer 24 and the orientation of the slot 28 are identical. The robot arm (not shown) is assumed to be perfectly manufactured and therefore the wafer 24 remains in the same plane during its transport to and from slot 28. Since the plane of motion of wafer 24, depicted in FIG. 2B, is coincident with the plane of the wafer itself (and more particularly with the object axis P) and the plane of the slot 28, no obstructions in the travel path are encountered and motion of the wafer 24 between the approach position and the pickup position is unhampered. For clarity, the approach position is defined with respect to the cassette and is to be understood as the position at which the end effector and/or end effector-wafer combination approach or retract from the cassette, while the pickup position is defined with respect to the wafer itself and is the position at which the end effector is just about to engage or disengage from the wafer.
In a first non-ideal situation encountered in practice and depicted in FIG. 3A, the orientation of the wafer 24 is different from that of slot 28 which it occupies, with object axis P being transverse to the axis of the slot 28. Since the wafer 24 cannot be withdrawn from the cassette 22 in this transverse position, either the cassette 22 must be rotated while the orientation of the wafer 24 is maintained until parallelism of the two is achieved (FIG. 3B), or the robot itself must be rotated while supporting wafer 24 (FIG. 3C). The rotation of the cassette is a disruptive intervention which must be performed manually and detracts from system throughput and efficiency, while rotation of the wafer-end effector combination can only be performed using GPR-type robots.
A worse situation, depicted in FIGS. 4A and 4B, occurs when the wafer 24, initially properly aligned within slot 28 (FIG. 4A), changes its orientation and vertical position during motion due to for example geometric inaccuracies of the arm. Equally undesirable is for the wafer 24 to approach the slot 28 in the displaced orientation and vertical position during the reverse, insertion process into the cassette 22. A manifestation of this is the tilt of object axis P with respect to the direction of motion a such that the orientation of wafer 24 is transverse to the direction of motion a.
FIGS. 6A-6C show the motion of a semiconductor wafer 24 during retraction from a slot 28 of a misaligned cassette 22. As can be seen from the drawing figure, because the direction of motion of wafer 24 is not coincident with object axis P of wafer 24, an object shadow 29 is created which the wafer 24, over the course of the transport duration, necessarily occupies. This object shadow 29 exceeds the slot width shown in FIGS. 6A and 6B and imposes the requirement of an expanded width on slot 28 as shown in FIG. 6C in order to permit unobstructed retraction or insertion of the wafer 24 into the cassette 22. Accordingly, wafer pitch and cassette capacity are reduced.
Conventional non-GPR robots cannot obviate this situation because they are unable to change the orientation of the end effector about its longitudinal axis and because they lack adequate algorithmic resources to implement the necessary combination of motions. GPR robots, on the other hand, can compensate for the undesirable deviations because these robots can for example be tilted along the Z axis, as shown in FIG. 5.
FIG. 5 schematically shows two positions of a wafer-carrying GPR robot: compensated position 30 and uncompensated position 30xe2x80x2. The compensation in this case is effected in order to maintain a horizontal position of the wafer 24, although other positions can of course also be achieved. As can be seen, in the uncompensated position 30xe2x80x2, with the arm 16xe2x80x2 extended, the position of the end effector 18xe2x80x2 and the wafer 24xe2x80x2 deviate from the horizontal, exhibiting a sag due to for example the weight of wafer 24xe2x80x2, arm 16xe2x80x2 and end effector 18xe2x80x2, and to the geometry and stiffness of arm 16xe2x80x2 and end effector 18xe2x80x2. In order to compensate for this deviation, platform 14 is tilted a predetermined angle xcex1 and lowered an amount dZ while arm 16 is extended by a predetermined amount. The resultant tilt re-aligns wafer 24 to a horizontal orientation.
Because the above situations are encountered in practice, it is desirable to control robot motion such that the robot arm is made to counter-act anticipated deflections and deviations from its intended path. In such a manner greater precision and substrate pitch are achieved and process throughput improved.
The present invention overcomes the deficiencies of the prior art by actively compensating for deviations in the travel path of the robot arm. In accordance with the invention, the velocity components of robot arm motion are synchronized during extension and retraction of the robot arm to thereby compensate for mechanical and other imperfections, as well as other imperfections of the arm and the manipulated object, which would otherwise cause deviations from an ideal path.
In accordance with the preferred embodiment, motion of the robot arm in the Z direction is synchronized with the planar motion of the robot arm such that the trajectory of the transported object, which may be the end effector of the robot arm itself, is along an object axis of the object. The synchronization may be a linear interpolation between the approach and pickup positions of the robot in terms of the end-effector coordinates. The effects of robot arm position deviation due to for example the weight of the substrate is canceled by applying a predetermined motion algorithm taking into account the position of the robot arm-object combination as well as their weight, along with other factors impacting the accuracy of the object manipulation process.
GPRs (Global Positioning Robots) are particularly well-suited to minimize such deviations because their triple actuated Z axis provides stronger support to the upper mechanical structure and because their kinematic versatility enables adjustment of the orientation of the platform of the GPR and the end effector to the orientation of the cassettes without affecting the characteristic point (center) of the manipulated substrate. In accordance with the invention, the dexterity of GPRs is exploited with synchronous movement of the platform during transition between the approach and pickup positions of the end effector associated with a cassette or another substrate holder.